1. Field of the Invention
This invention relates to load indicating members and, more particularly, to load indicating members, such as fasteners, having ultrasonic transducers.
2. Description of the Prior Art
In many operations, it is desirable to determine the amount of longitudinal load experienced by a longitudinally stressed member. This information is particularly useful when the longitudinally stressed member is a fastener since the measurement of the longitudinal stress provides a verification of the existence of a proper joint.
Many prior art techniques have been developed to indicate the amount of longitudinal stress experienced by a fastener by providing a load indicating feature to the fastener itself. This is usually done by interconnecting one end of an elongated member, such as a pin, to a portion of the fastener prior to the installation of the fastener. While each of the various pin-type load indicating members, and load measuring devices known in the art provides its own advantages in terms of accuracy, ease of manufacture, or ease of reading, they are still expensive to manufacture, since they each require extensive modifications and the addition of a centrally located pin-member to the load indicating member. As a result, such load indicating members are only selectively used in practice, either where there is a specific immediate diagnostic need or where there is a serious recognized safety hazard involved. These members are simply too expensive for routine use in assemblies which may only occasionally benefit from such monitoring.
An alternative approach to measuring the elongation of a member or fastener is to use an ultrasonic measurement device. Typically, this is done by removeably interconnecting an ultrasonic transducer to one end of the member being measured, usually to the head of a fastener. In order to obtain a reliable indication, the head of the bolt must be ground extremely flat and a reliable ultrasonic transmission media must be applied to the head of the bolt. The transducer must be properly positioned on the bolt and held in position while the measurements are being taken. Various examples of techniques and apparatuses using this method are known in the art. Furthermore, the prior art teachings include the notion of combining the measuring device with a tightening tool so that the information gained from measuring the elongation of the bolt can be used for determining when to shut off the tightening tool or, alternatively, for monitoring the tightening process to determine whether a proper joint has been formed.
While the above-mentioned methods and apparatuses can provide reliable information about a fastener and a joint, they are in very limited use. This is mainly because the bolt must be carefully manufactured and must be easily accessible to the instrumentation. Thus, ultrasonic tension measurement is recognized as a highly accurate laboratory tightening method for calibration, application testing and for tightening very critical joints. It is replacing strain gage bolts in several calibration and critical quality control applications. However, practical difficulties associated with taking ultrasonic tension measurements have prevented its application as a general assembly tightening strategy. These practical difficulties include: difficulty in maintaining reliable acoustic coupling during tightening; difficulties presented by equipment expense and complexity; and difficulties presented by experimental determination of parameters for each joint.
Suggestions of the prior art for ultrasonic load indicating-type members utilizing a piezoelectric or other ultrasonic sensor include: U.S. Pat. No. 4,294,122, Couchman, which discloses a threaded bolt having an opening in either its head or its opposite end with an ultrasonic transducer fixedly secured therein for use in obtaining preload measurements, as well as other measurements for quality control inspection or for monitoring purposes. Also, U.S. Pat. No. 4,569,229, de Halleux, which discloses a method for measuring strains in a part including the steps of: selecting within a medium one or a plurality of ends of rectilinear measuring runs, which ends are embodied by an inner artificial reflector; emitting a beam of acoustic waves so that acoustic rays carrying sufficient energy strike the useful reflectors; selecting the echoes corresponding to the reflectors; determining by measurement the transit times which are characteristic of the useful acoustic rays up to the inner artificial reflectors and transposing the transit times for each inner reflector considered individually or the respective differences of transit times for each couple of reflectors into an outer strain value or into a strain value within the region delimited by each couple of reflectors. The part for implementing such a method has reflectors consisting particularly of perforations or bores into the part.
Additionally, U.S. Pat. No. 4,601,207, Steblay, discloses a mine roof bolt and a method of measuring the strain in mine roof bolts wherein a flat portion on the head of the mine roof bolt is first machined, and then a hole is drilled radially through the bolt at a predetermined distance from the bolt head. After installation of the mine roof bolt and loading, the strain of the mine roof bolt is measured by generating an ultrasonic pulse at the flat portion. The time of travel of the ultrasonic pulse reflected from the hole is measured. This time of travel is a function of the distance from the flat portion to the hole and increases as the bolt is loaded. Consequently, the time measurement is correlated to the strain in the bolt. Compensation for various factors affecting the travel time are also provided.
Furthermore, U.S. Pat. No. 4,846,001, Kibblewhite, teaches the use of a thin piezoelectric sensor consisting of a piezoelectric film sandwiched between two thin electrodes, which is permanently, mechanically and acoustically coupled to the upper surface of a member and is used to determine the length, tensile load, stress, or other tensile load dependent characteristic of the member by ultrasonic techniques.